The invention relates to a method for generating electrical energy in a combined energy generation plant that comprises an air treatment unit and a power plant unit, and a corresponding energy generation plant which comprises an air treatment unit and a power plant unit, wherein                in a first operating mode, in the air treatment unit, air is successively compressed, cooled and expanded and used for obtaining an air liquefaction product,        in a second operating mode, in the air treatment unit, an air liquefaction product is vaporized or pseudo vaporized at superatmospheric pressure and used for obtaining electrical energy in the power plant unit, and        in a third operating mode, in the air treatment unit, air is compressed and used in the power plant unit for obtaining electrical energy.        
The energy generation plant for generating electrical energy has an air treatment unit combined with a power plant unit, which air treatment unit comprises a compressor arrangement, a heat-exchange system having a coolant system, a liquefaction system and a tank system and is equipped,                in a first operating mode to compress air in the compressor arrangement, to cool it in the heat-exchange system and to expand it in the liquefaction system and to use it for obtaining an air liquefaction product,        in second operating mode to vaporize or pseudo vaporize an air liquefaction product in the heat-exchange system at superatmospheric pressure and to use it in the power plant unit to obtain electrical energy, and        in a third operating mode to compress air in the compressor arrangement and to use it in the power plant unit to obtain electrical energy.        
It is known, for example from DE 31 39 567 A1 and EP 1 989 400 A1, to use liquid air or liquid nitrogen, that is to say low-temperature air liquefaction products, for grid control and for providing controlling power in electricity grids.
At cheap electricity times, or electricity surplus times, in this liquefied in an air separation plant with an integrated liquefier or in a dedicated liquefaction plant, here generally termed air treatment unit, in total or in part to give such an air liquefaction product. The air liquefaction product is stored in a tank system using low-temperature tanks. This operating mode is here termed “liquefaction mode”.
At peak load times, the air liquefaction product is taken off from the tank system, boosted in pressure by means of a pump and warmed to about ambient temperature or above, and thereby transformed into a gaseous or supercritical state. A high-pressure stream obtained thereby is expanded to ambient pressure in a power plant unit in an expansion turbine or a plurality of expansion turbines with intermediate heating. The mechanical power released in this case is converted into electrical energy in one or more generators of the power plant unit and fed into an electrical grid. This mode of operation is here termed “take off mode”.
Corresponding methods and devices can, as can also the method and device of the invention, in principle also operate with an air liquefaction product that contains more than 40 mol % oxygen. However, this has been excluded here in order to avoid confusion with methods and devices in which a particularly oxygen-rich fluid is introduced into a gas turbine to support oxidation reactions.
The cold released on transformation of the air liquefaction product into the gaseous or supercritical state can also be stored during the take off mode and used during the liquefaction mode for providing cold for obtaining the air liquefaction product.
Finally, compressed air storage power plants are also known in which the feed air is not, however, liquefied, but is compressed in a compressor and stored in a subterranean cavern. In times of high electricity demand, the compressed air is passed out of the cavern into the combustion chamber of a gas turbine. At the same time, fuel, for example natural gas, is fed to the gas turbine via a gas line, and burnt in the atmosphere formed by the compressed air. The exhaust gas formed is expanded in the gas turbine, as a result of which energy is generated.
The economic efficiency of corresponding methods and devices is greatly affected by the overall efficiency. The object underlying the invention is to improve corresponding methods and devices in their economic efficiency.